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DISTRIBUTION CATEGORY UC-59
Architectural and Engineering
Solar Systems Design
July 15, 1978
Work Performed Under Contract No. EG-77-C-01-4049
U.S. Department of Energy
National Solar Heating and
Cooling Demonstration Program
National Solar Data Program
This report was prepared as an account of work sponsored by the United States
Government. Neither the United States nor the United States Department of Energy, nor
any of their employees, nor any of their contractors, subcontractors, or their employees,
makes any warranty, express or implied, or assumes any legal liability or responsibility for
the accuracy, completeness or usefulness of any information, apparatus, product or process
disclosed, or represents that its use would not infringe privately owned rights.
This report has been reproduced directly from the best available copy.
Available from the National Technical Information Service, U. S. Department of
Commerce, Springfield, Virginia 22161.
Price: Paper Copy S4.00
A key part of the national solar demonstration program is the acquisition and dissemina-
tion of data generated by the demonstration projects. The Ehrenkrantz Group, Architects
and Planners, as a subcontractor to PRC Energy Analysis Company under contract to the
Department of Energy, has been involved in this process for more than a year. The
following paper, based on the review and analysis of over 70 projects, will focus on some
of the most common design and construction problems. Structural details and specifica-
tions and some trade-offs that can make for a more cost-effective project will be
addressed rather than mechanical considerations, such as collector selection.
THE SOLAR DESIGNER
IS WORKING WITH
LEVELS OF THERMAL
THIS LIMITS THE AMOCINT
OF MONEY THAT CAN
EXPENDED ON THE
TRANSPORT OF SOLAR
The whole solar installation must be
value-engineered to arrive at the lowest
first cost consistent with quality of mate-
rials and workmanship required. The
desire to extract every theoretical Btu
from a system often leads to the installa-
tion of extra devices that will not be
cost-effective and will only increase the
payback period of the project.
If we look out over a typical residential or
commercial building roofscape we see a
minimal amount of exposed construction,
especially piping. As a result, the build-
ing trades , cannot be expected to bring
prior experience to bear in solving typical
problems that accompany solar projects
with their large amount of exposed exter-
SOLAR COLLECTION SYSTEMS
CONSTRUCTION THAN THE
BUILDING TRADES ARE
THEREFORE, THE DESIGNER
MUST ISOLATE AND SOLVE
AND NOT LEAVE DECISIONS
TO THE CONTRACTOR.
Layout and Access vs. Cost— Some considerations that have an impact on costs involve the
collectors and structural layouts and the degree of accessibility provided to the collectors
and piping for servicing. The designer feels that immediate access to all parts of the
array is mandatory, especially in the case of evacuated tube designs. One of the more
handsome designs we have seen provides accessibility to each row of collectors.
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Each row of collectors is slightly higher than the one in front of it, with accessibility to
piping and collectors in the form of a walkway at each location. This makes the system
both extremely accessible and extremely expensive. The large amount of miscellaneous
metal that goes into this design is very expensive and has delayed the project.
Simpler and less expensive approaches would extend the array to the full extent of the
available space provided by the structure, simplify the design of the collector support, and
eliminate the walkways.
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One can see that by changing the level of the structure only slightly, it would be possible
to simplify the installation.
Another approach would be to double up the last two rows of collectors and keep the
In either case, by keeping the whole structure near the roof, accessibility could have been
provided with a ladder rather than with walking surfaces.
I like to call the next design the "O.S.H.A. Special." Notice that ladders have been
provided as well as walkways and handrails. Again, this is extremely expensive. In this
particular project, which was built by Unistrut, the structure is much too flexible, and the
collector will leak if there is any movement in the structure.
A much less expensive, faster, and easier
alternative would have been to place two
single rows of collectors (as noted in the
circle) on the roof— assuming there is suf-
ficient space available. The simpler
structure would be sufficiently rigid, and
access would be from the roof.
Cost of Space Frames and Special
Shapes vs. Standard Steel Shapes—
Another cost trade-off involves the
design of the collector support. It
seems that when the requirements for
triangular support and fairly long
spans are presented, the first things
that come to the designer's mind are
special shapes and space frames. In
one case, it's an aluminum tube space
frame; in another case, it's a square
tube steel frame. Both will be hand-
some structures, if one can afford to
build them. However, a much more
efficient and less expensive approach
is the use of simple, rolled-steel, angle
sections with steel beams for the ma-
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Support Trade-offs — Col lec-
tor supports ultimately must
be tied down to the roof
structure. Designers have
decided, in some cases to
try to avoid penetrating the
roofing. In this illustration,
you see an attempt to span
between a wall and a para-
pet to avoid penetrating the
roofing. After a review, it
was discovered that $40,000
could be saved by putting in
two pitch pockets 12.5 feet
on center, rather than a 27
WF steel beam 12 feet on
The structure presented in
the adjacent illustration
could not take any point
loads. As a result, the de-
signers tried to distribute
the load over the whole
structure by using 700 pitch
pockets. The probability
that this structure will leak
is quite high.
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Once you have decided on your basic
structural approach, the next question is
how to anchor it down. You can use
regular pitch pockets, curbs constructed
in the field, manufactured curbs or (if
your vertical support is c regular shape,
such as square or rouno,/ a neoprened
sleeve. There is also a simple sleeper
that can be bolted down.
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In some designs there has been an attempt to minimize or eliminate roofing penetrations.
For example, in this case the designer was
working with a light frame structure,
using sleepers not attached to the roof
structure and guy wires to anchor every-
thing in place.
In the adjacent illustration, the structural
members span between available vertical
supports. Although it was not economical
in this case, it can be. If you use this
approach, be sure to allow for tempera-
Another interesting approach involved the
use of heavy dead-load concrete blocks,
tied together with structural members, to
withstand the horizontal force of the
wind. If your roof can take it, this is a
feasible way to go, but you'll pay a
penalty in the extra structure reguired to
handle the dead load.
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Flashing and Drainage Problems— Remember that flat roofs are not flat; they pitch for
drainage. If you decide to use sleepers or continuous curbs, there's a good chance that you
will be cutting off the roof's natural drainage. Be sure that your roof can take puddling.
Some roofing materials can and will deteriorate under puddled water conditions.
If you are doing a retrofit, check
the existing material. In a new
installation, be sure that the cor-
rect roofing material is specified.
There is a tendency to put supports close
to the roof edge to utilize as much sur-
face as possible; this results in interfer-
ence between the two sets of flashing.
Allowances in detailing can correct this
Shim Requirements- -! have already noted
that flat roofs are not flat. They either
pitch to drains or, in long spans, the
beams are cambered. Therefore, the de-
sign must allow for shimming of the
structure. Do not leave it to the con-
tractor. If you do not detail and specify
the location and type, you may have
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Snow Loads, Roof Maintenance, Drain Down— These items may seem unrelated; however,
they should all be considered prior to establishing final collector support design.
Snow Loads— If collectors
are installed close to the
roof, they will not only act
as a snow fence but will
actually cause additional
snow accumulation. Besides
carrying an extra load,
buried collectors obviously
are not going to work very
well for the first week or
two. If you lift them above
the roof and let the wind do
the shoveling, you can solve
a lot of maintenance prob-
lems, lower your snow load,
and improve the efficiency
of your collector.
Also, if your collectors ex-
tend over an entry or a
walkway, please protect the
pedestrians. I think that the
pedestrian and your client
will appreciate this protec-
tion and so will OSHA.
Roof Maintenance— The higher you keep
the collectors, including the structure and
pipes, the easier it will be to maintain,
repair, or even replace the roof in the
future. If you provide walking surface
accessways for maintenance personnel
(with or without ladders), you will find
that it will save much wear and tear on
the roof. Collectors are often installed at
a very shallow angle (especially in the
South to best accommodate cooling
loads). Limited space creates repair
problems. The condition illustrated on
the left would be a real "bear" to con-
Drain Down— When design-
ing a draindown system, it is
very common to draw your
collector support and note
on the drawings or in the
specifications that the pipes
must be "pitched to drain."
In the case illustrated at
right, the designer drew the
collector without leaving
enough room to install the
pipe itself— let alone pitch
it to drain. If you want the
collectors and piping to
pitch to the drain, be sure
you detail it so that they
can. Either pitch the steel
or lift up the collectors so
that the pipes can pitch. In
addition, most people would
like to avoid having pitching
pipes all over the place. It
does create a vertical
problem in installing collec-
tors. Many designers,
therefore, are tempted to
design the piping with 1/8
inch pitch per foot. The
allowable roof deflection on
a normal beam is 1/10 inch
per foot; therefore, the pipe
pitching at certain parts of
that span will be 1/40 inch.
Normally, we call this a
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Structural Loads— The loads that are imposed
by solar installations are fairly well known.
There is the dead load, the wind load (with its
horizontal and vertical components), and the
snow load. These loads can vary depending
upon your design. Because of the extent of
exterior construction, we have a new load-
temperature expansion and contraction due
to temperature differential.
The structure could experience a differential
temperature of 160 F between the summer
sun and the cold of a winter night, and your
piping could experience a 200 F differential.
We must, therefore, address the considerable
problem of thermal expansion and con-
We have found few problems involving the actual structural support of the collector loads;
however, the architectural implications of these loads sometimes go unnoticed.
For example, if you fasten a post
and base plate to a wood curb, you
will have problems if you have only
metal fasteners holding the metal
base plate through metal flashings.
Metal to metal to metal will un-
doubtedly allow water to enter
through the holes caused by the
fasteners. In addition to the water
problem, the back and forth motion
caused by wind forces and temper-
ature differentials actually can
cause these fasteners to pull out
slightly. If your wood is not treat-
ed, it can deteriorate and cause
futher problems. Also, as this post
rocks, the base plate will tend to
cause metal fatigue to the cap
flashing at its edges.
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In the next case, the design called
for a prefabricated curb bolted to
the structure with a beam over it.
When the beam moves back and
forth, either because of wind load,
expansion and contraction, or both,
there will tend to be movement of
the curb where it joins the roofing
that could cause a roofing failure.
An easy way to avoid this problem
is to be sure you specify that the
edges of the curb be secured to the
roof structure. Also, when detail-
ing wood curbs for construction in
the field, be sure that you provide
sufficient lateral strength.
A very common problem is the use
of sleepers on a retrofit so as not
to disturb the existing roofing— the
sleeper is merely bolted down on
top of the roofing. If you put a
load onto a sleeper, over normal
insulation, the normal insulation
will not be able to take the com-
pressive loads that can be applied.
The sleeper will compress the in-
sulation and actually shear the
waterproofing membrane along the
edge of the sleeper, thereby
We can see that there is more than one way to cause a leak. If there is one thing we
would prefer to avoid, it is having solar buildings become synonymous with leaking
1. ITS SPECIAL
2. ITS SPECIAL
Before making the final selection of a collector,
be sure that you understand all the attributes of
your collector. In addition to such mundane
characteristics as collector efficiency and
stagnation temperatures, you must consider
other truly important characteristics, e.g.,
attachment requirements and rigidity of
Insulation — Rigid fiberglass is a very com-
mon insulating material. It is relatively
low in cost, is available in sizes that vary
up to 3 inches, and it resists high heat. It
absorbs water, however, and if water does
get in, you will have a radiator— not an
insulator. Also, it cannot take the
flexible shape very often required in solar
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Rigid foam is comparable in cost to fiber-
glass for the same R value. Similar to
glass, it too varies up to about 3 inches.
However, it deteriorates very badly if
exposed to high heat associated with stag-
nation. If that is a problem, you might
consider some sort of pipe wrap as an
inner protection. Pipe wrap is not readily
affected by water, but it is not water-
proof —you cannot use it as waterproofing.
As a matter of fact, water coming in
contact with some rigid foams and pipe
will cause a mild acid that will deterio-
rate the pipe. So, it must be water-
proofed effectively. Also, rigid foam
cannot take the flexible shapes that we
encounter with solar.
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What are the special instal-
lation requirements of your
collector? One problem,
mentioned earlier, is that
any movement in the struc-
ture (which must be rigid)
will cause the collector to
Other collectors require
special hardware mounting.
Some collectors can only be
tilted in one direction, or
they will air bind.
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Others have waterproofing
problems— especially where
the pipes extend out of the
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Sometimes collectors have
special capabilities. Among
these special capabilities is
the ability to be fastened in
very particular ways, e.g.,
at the corner. If your col-
lector is capable of SDanning
and being fastened at dif
ferent places, it simplifies
your structure. You do not
have to have a member in
an exact location; you have
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Other collectors are quite
long and require very little
support because they are
sufficiently rigid to take
lateral movement. There-
fore, put as little structure
behind them as possible and
use the strength of the col-
For a successful project, provide for the
collectors' requirements and use their
capabilities. Whatever your collector
problems are, you must know what mater-
ials comprise your collector frame, sup-
port structure, and fasteners. If there are
any differential metals involved, there
should be nonmetallic materials separa-
ting them to prevent galvanic action.
Considering that most cooling equipment
requires liquids in the temperature range
of 180-200 F, and that most collectors
are very hard put to supply that type of
energy, any loss due to piping or storage
can make the whole system nonoperative.
TILT OR ORIENTATION
OF 10°± WILL AFFECT THE
TOTAL SYSTEM PERFORMANCE
BY LESS THAN 10%.
A POOR PIPING INSTALLATION
CAN AFFECT THE TOTAL
BY MORE THAN 50%.
Insulations —Most insulation materials were not developed for use in solar projects.
Although many materials are being adapted and used, they are not suitable in all cases.
Let's look at some of the problems.
First, flexible foam is only available in
1/2-inch or 3/4-inch thicknesses. To
increase your R value you must add a
second layer, thereby, doubling your cost.
Second, the foam is affected by high heat.
(The use of pipe wrap could help in this
area.) Third, it is affected by the sun's
ultra-violet rays. (It can be painted to
protect against ultraviolet deterioration.
If you do not paint it, it may deteriorate
badly in six months and possibly fail
completely within a year.) On the plus
side, flexible foam is waterproof and is
capable of taking any shape you wish.
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Pipe Supports -- It is very important to decide
how to support your pipes within a solar
Pipes can be supported off the collector. This is
normally done by using soldered copper nipples.
Be very careful that the mechanic does not burn
out the waterproof seals of the collector. This is
a constant problem because the required solder
is of a particularly high temperature.
Pipes can also be supported off the collector
support. Be sure you have designed space on the
collector support to support the piping. You
might consider the alternative of supporting the
pipes off the back of the support.
The pipes can also be supported on the roof
surface, but try to design the pipes so that the
workmen do not use them as a platform.
A fairly common way to support pipes is off
other pipes, especially in reverse-return designs.
In this case, be sure that the pipes are well
supported, one from the other, and that they are
both free to move.
Another common pipe detailing problem is that when pipes are shown as lines on paper,
nobody worries about the fact that they cross. Remember that pipes have thicknesses,
usually 3 inches or more, and you must account for this thickness in your design. Also, be
sure that you design enough space between collectors to allow for the installation of
valves or expansion compensators if needed.
Pipe Movement — We have shown that
there could be as much as 2 to 2 1/2
inches of movement in the pipe, and it is
certain that they are always in opposite
directions or at right angles. So, always
allow for this pipe movement at change of
direction or in extremely long lengths.
Some of the means of compensating for
these movements include silicone bulbs,
bellows, braided wire or elbow configura-
tions to the piping. Whatever method you
choose, design it and show it.
VVaterproofing --Frequently, waterproofing
problem areas occur between collectors
and piping at joints between pipes, change
of direction in pipes or pipe T's, or where
valves or monitoring equipment protrude
through the waterproofing. Be sure that
your design answers these problems. This
is especially important with fiberglass
because any water getting into it will
lower the efficiency of the whole system
Do not allow open seams.
Provide joint protection at
all elbows and tees.
Do not bind your pipes when
supporting them. Be sure
that the pipes are free to
move, or, with pipe move-
ment, you will tear either
your waterproofing or your
Do not support piping by
puncturing your waterproof-
ing, especially in terms of
supporting the pipe directly.
Puncturing not only destroys
the waterproofing and
wreaks havoc with the insu-
lation but also allows a heat
bridge to radiate additional
energy. Support the pipe
loosely on the outside of the
Do not seal the ends of
sheet waterproofings. Seal
between the seams or the
sealant will fail with move-
ment. End seals will fail;
seam seals tend not to fail.
Lastly, avoid differential
metals if you are using alu-
minum waterproofing. Be
sure that you have nonme-
tallic protection to prevent
galvanic action if you have
steel structure or steel
clamps. We saw one very
attractive installation that
had aluminum waterproofing
and a sheet of galvanized
steel under it with no separ-
ation protection. That wa-
terproofing will fail.
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FOR SOLAR PROJECTS
TO BE COST-EFFECTIVE THE
ISOLATE THE DIFFERENT
COST TRADE-OFFS AND
CHOOSE THE MOST
• THE A/E MUST ISOLATE ALL
• THE A/E MUST ALSO SOLVE
• DETAILS AND SPECIFICA-
TIONS MUST ADDRESS
• THE A/E MUST FOLLOW UP
WITH SUFFICIENT FIELD
• DON'T LEAVE PROBLEMS
FOR THE CONTRACTORS
We have seen that there is a lot more to designing an effective solar-assisted mechanical
system than just choosing your collector area and mechanical equipment; a successful
project demands a high level of attention to all details.
UNIVERSITY OF FLORIDA
3 1262 09052 5261